WO2018134874A1 - ホットスタンプ成形体及びその製造方法 - Google Patents
ホットスタンプ成形体及びその製造方法 Download PDFInfo
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- 238000004519 manufacturing process Methods 0.000 title claims abstract description 26
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 112
- 239000010959 steel Substances 0.000 claims abstract description 112
- 238000010438 heat treatment Methods 0.000 claims abstract description 103
- 239000000463 material Substances 0.000 claims abstract description 47
- 238000010791 quenching Methods 0.000 claims description 76
- 230000000171 quenching effect Effects 0.000 claims description 76
- 238000001816 cooling Methods 0.000 claims description 60
- 239000002245 particle Substances 0.000 claims description 57
- 229910000734 martensite Inorganic materials 0.000 claims description 51
- 238000000034 method Methods 0.000 claims description 15
- 229910001566 austenite Inorganic materials 0.000 claims description 9
- 238000005496 tempering Methods 0.000 abstract 7
- 208000013201 Stress fracture Diseases 0.000 description 47
- 238000012360 testing method Methods 0.000 description 34
- 239000000047 product Substances 0.000 description 30
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 22
- 229910001563 bainite Inorganic materials 0.000 description 21
- 238000000137 annealing Methods 0.000 description 20
- 150000001247 metal acetylides Chemical class 0.000 description 18
- 229910000859 α-Fe Inorganic materials 0.000 description 15
- 238000007747 plating Methods 0.000 description 13
- 230000000694 effects Effects 0.000 description 12
- 229910001562 pearlite Inorganic materials 0.000 description 12
- 239000010960 cold rolled steel Substances 0.000 description 11
- 230000009466 transformation Effects 0.000 description 11
- 238000005246 galvanizing Methods 0.000 description 10
- 229910052742 iron Inorganic materials 0.000 description 10
- 239000013078 crystal Substances 0.000 description 8
- 239000012535 impurity Substances 0.000 description 8
- 239000000203 mixture Substances 0.000 description 8
- 230000035882 stress Effects 0.000 description 8
- 238000009864 tensile test Methods 0.000 description 8
- 208000010392 Bone Fractures Diseases 0.000 description 7
- 206010017076 Fracture Diseases 0.000 description 7
- 238000005485 electric heating Methods 0.000 description 7
- 239000000126 substance Substances 0.000 description 7
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 6
- 238000005097 cold rolling Methods 0.000 description 6
- 230000009467 reduction Effects 0.000 description 6
- 239000000523 sample Substances 0.000 description 6
- 229910052725 zinc Inorganic materials 0.000 description 6
- 239000011701 zinc Substances 0.000 description 6
- 238000002474 experimental method Methods 0.000 description 5
- 238000005098 hot rolling Methods 0.000 description 5
- 229910052758 niobium Inorganic materials 0.000 description 5
- 230000000717 retained effect Effects 0.000 description 5
- 238000005728 strengthening Methods 0.000 description 5
- 229910052720 vanadium Inorganic materials 0.000 description 5
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 5
- 229910001335 Galvanized steel Inorganic materials 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- 229910052802 copper Inorganic materials 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 239000008397 galvanized steel Substances 0.000 description 4
- 230000006872 improvement Effects 0.000 description 4
- 229910052759 nickel Inorganic materials 0.000 description 4
- 229910052719 titanium Inorganic materials 0.000 description 4
- 238000011282 treatment Methods 0.000 description 4
- 230000009471 action Effects 0.000 description 3
- 238000005275 alloying Methods 0.000 description 3
- 229910052804 chromium Inorganic materials 0.000 description 3
- 238000007796 conventional method Methods 0.000 description 3
- 229910052751 metal Inorganic materials 0.000 description 3
- 239000002184 metal Substances 0.000 description 3
- 229910052750 molybdenum Inorganic materials 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 238000005096 rolling process Methods 0.000 description 3
- 229920006395 saturated elastomer Polymers 0.000 description 3
- 238000005204 segregation Methods 0.000 description 3
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 2
- 229910001297 Zn alloy Inorganic materials 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 229910052787 antimony Inorganic materials 0.000 description 2
- 229910052785 arsenic Inorganic materials 0.000 description 2
- 229910052796 boron Inorganic materials 0.000 description 2
- 239000005539 carbonized material Substances 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 238000005520 cutting process Methods 0.000 description 2
- 238000002149 energy-dispersive X-ray emission spectroscopy Methods 0.000 description 2
- 238000000445 field-emission scanning electron microscopy Methods 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 229910052739 hydrogen Inorganic materials 0.000 description 2
- 239000001257 hydrogen Substances 0.000 description 2
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 238000000465 moulding Methods 0.000 description 2
- 238000010899 nucleation Methods 0.000 description 2
- 230000006911 nucleation Effects 0.000 description 2
- 238000005554 pickling Methods 0.000 description 2
- 238000004080 punching Methods 0.000 description 2
- 238000005507 spraying Methods 0.000 description 2
- 229910052715 tantalum Inorganic materials 0.000 description 2
- 229910052718 tin Inorganic materials 0.000 description 2
- 229910052721 tungsten Inorganic materials 0.000 description 2
- 230000000007 visual effect Effects 0.000 description 2
- 238000004804 winding Methods 0.000 description 2
- 229910052727 yttrium Inorganic materials 0.000 description 2
- 229910052726 zirconium Inorganic materials 0.000 description 2
- 238000002441 X-ray diffraction Methods 0.000 description 1
- 229910000611 Zinc aluminium Inorganic materials 0.000 description 1
- HXFVOUUOTHJFPX-UHFFFAOYSA-N alumane;zinc Chemical compound [AlH3].[Zn] HXFVOUUOTHJFPX-UHFFFAOYSA-N 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 238000001389 atom probe field ion microscopy Methods 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- -1 but in general Substances 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000005238 degreasing Methods 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000005868 electrolysis reaction Methods 0.000 description 1
- 238000009713 electroplating Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 229910000765 intermetallic Inorganic materials 0.000 description 1
- KFZAUHNPPZCSCR-UHFFFAOYSA-N iron zinc Chemical compound [Fe].[Zn] KFZAUHNPPZCSCR-UHFFFAOYSA-N 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- QELJHCBNGDEXLD-UHFFFAOYSA-N nickel zinc Chemical compound [Ni].[Zn] QELJHCBNGDEXLD-UHFFFAOYSA-N 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 238000007493 shaping process Methods 0.000 description 1
- 238000002791 soaking Methods 0.000 description 1
- 230000001629 suppression Effects 0.000 description 1
- 230000008646 thermal stress Effects 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/62—Quenching devices
- C21D1/673—Quenching devices for die quenching
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/02—Hardening by precipitation
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a hot stamping molded body and a manufacturing method thereof.
- a method for forming a high-strength steel plate As a method for forming a high-strength steel plate, a method called hot stamping is known.
- hot stamping a steel sheet having a C content of about 0.20 mass% to 0.22 mass% is press-molded at a high temperature range of 700 ° C. or higher, and is quenched in or outside the press mold. According to the hot stamping, since the forming is performed in a high temperature range where the strength of the steel sheet is reduced, it is possible to suppress a forming defect that occurs in a cold press.
- a low-stress fracture may occur in a hot stamped molded body having a tensile strength of 1900 MPa or more.
- a hot stamped molded body that causes low stress fracture is used for a structural part for automobiles, the part may be broken even when subjected to a calculated impact that can be withstood in the design stage. Therefore, suppression of low stress fracture is extremely important for ensuring collision safety of automotive structural components. So far, low-stress fracture of marage steel is known, but low-stress fracture of hot stamped compacts is not known.
- JP 2012-41613 A JP 2014-156653 A Japanese Patent No. 5756773 JP 2014-118613 A Japanese Patent No. 5402191
- An object of the present invention is to provide a hot stamping molded body that is high in strength and can suppress low stress fracture, and a manufacturing method thereof.
- the inventors of the present invention have studied to elucidate the cause of low stress fracture in a hot stamping molded body having a tensile strength of 1900 MPa or more.
- Equation 1 a material that is broken before the following Equation 1 is satisfied undergoes low stress fracture. It is called a material, and a material that occurs after Equation 1 is satisfied is called a material that does not cause low stress fracture.
- ⁇ true stress
- ⁇ true strain
- Equation 1 is the maximum load condition derived from the constant volume law during deformation.
- d ⁇ / d ⁇ is larger than ⁇ immediately after the start of the tensile test, and as the deformation progresses, d ⁇ / d ⁇ decreases and ⁇ increases.
- the load is maximized at the moment when d ⁇ / d ⁇ becomes equal to ⁇ , and thereafter, the tensile test piece is constricted, so the load is reduced.
- a material where low stress fracture occurs fracture occurs before necking occurs in the tensile specimen, that is, when d ⁇ / d ⁇ is greater than ⁇ .
- the present inventors first investigated the relationship between the structure of the hot stamped product and low stress fracture. As a result, it was found that the smaller the old ⁇ grains and the smaller the coarse carbides, the less likely the low stress fracture occurs.
- the present inventors examined improvement of the structure of the steel sheet used for hot stamping in order to achieve both the refinement of the old ⁇ grains of the hot stamping compact and the reduction of coarse carbides.
- the hardness of the steel plate whose main phase is fresh martensite and tempered martensite is approximately the same as the hardness after hot stamping, that is, the hardness of the hot stamped compact. Since the Vickers hardness of the hot stamped molded product having a tensile strength of 1900 MPa is about 550 Hv, when trying to obtain a hot stamped molded product having a tensile strength of 1900 MPa or higher, the Vickers hardness of the steel sheet is about 550 Hv or higher. When manufacturing a hot stamping body, blank plates are formed by blanking steel plates by shear cutting or punching before hot stamping, and blanking of steel plates having a Vickers hardness of 550 Hv or more is extremely difficult.
- the present inventors conducted further intensive studies. As a result, the present inventors have found that by performing quenching at least twice under a predetermined condition after blanking, a hot stamp molded body having a new structure and having excellent fracture characteristics can be obtained. And based on such knowledge, it came up with the aspect of the invention shown below.
- the second quenching step includes a step of cooling the blank material at an average cooling rate of 20 ° C./second from 700 ° C. to a fifth temperature of Ms point ⁇ 50 ° C.
- Fresh martensite and tempered martensite area fraction 80% or more in total
- Old austenite particle size 20 ⁇ m or less
- carbide average particle size 0.5 ⁇ m or less
- C content is 0.27 mass% or more and 0.60 mass% or less
- the hot stamping molded body according to the present embodiment is represented by the area fraction of fresh martensite and tempered martensite: 80% or more in total, prior austenite particle size: 20 ⁇ m or less, and average particle size of carbide: 0.5 ⁇ m or less. It has a steel structure.
- a hot stamping molded body is a molded body obtained through hot stamping.
- Fresh martensite and tempered martensite contribute to the improvement of strength.
- the area fraction of fresh martensite and tempered martensite is less than 80% in total, sufficient strength, for example, tensile strength of 1900 MPa or more cannot be obtained. Therefore, the area fraction of fresh martensite and tempered martensite is 80% or more in total.
- the mechanical properties of the material depend on the volume fraction of the structure or phase, but if the steel structure is isotropic, the volume fraction is equivalent to the area fraction. The area fraction can be measured more simply than the volume fraction. Therefore, the area fraction is used in the present application.
- the old ⁇ particle size is the average particle size of the old ⁇ particles. If the old ⁇ grain size exceeds 20 ⁇ m, sufficient fracture toughness cannot be obtained, and low stress fracture tends to occur. Therefore, the old ⁇ particle size is 20 ⁇ m or less. From the viewpoint of improving fracture toughness and suppressing low stress fracture, the old ⁇ grain size is preferably 15 ⁇ m or less, more preferably 10 ⁇ m or less.
- the average particle size of the carbide is 0.5 ⁇ m or less. From the viewpoint of suppressing low stress fracture, the average particle size of the carbide is preferably 0.3 ⁇ m or less.
- the carbide includes iron-based carbides such as cementite and ⁇ carbide, and carbonitrides.
- Common steel structures include, for example, ferrite, pearlite, upper bainite, lower bainite, retained austenite, fresh martensite or tempered martensite, or any combination thereof.
- ferrite ferrite
- pearlite upper bainite
- lower bainite retained austenite
- fresh martensite fresh martensite or tempered martensite, or any combination thereof.
- an example of a method for measuring the area fraction of these structures or phases will be described.
- a sample is taken from the steel sheet using a cross section parallel to the rolling direction and parallel to the thickness direction as an observation surface.
- the observation surface is polished, nital etched, and the range from the depth of the steel plate to the depth of t / 8 to the depth of 3t / 8 when the thickness of the steel plate is t is an electrolytic radiation type at a magnification of 5000 times.
- FE-SEM scanning electron microscope
- each area fraction of ferrite, pearlite, upper bainite, lower bainite, and tempered martensite is obtained from the average value of 10 visual fields.
- the upper bainite, the lower bainite, and the tempered martensite can be distinguished from each other by the presence and absence of iron-based carbides in the lath-like crystal grains and the elongation direction.
- Upper bainite is a collection of lath-like crystal grains, and contains carbides between the laths.
- Lower bainite is an aggregate of lath-like crystal grains, and contains iron-based carbide having a major axis of 5 nm or more inside.
- the iron-based carbide contained in the lower bainite has a single variant, and the iron-based carbide existing in one crystal grain extends substantially in a single direction.
- substantially single direction means a direction in which the angle difference is within 5 °.
- Tempered martensite is an aggregate of lath-like crystal grains and contains iron-based carbide having a major axis of 5 nm or more inside.
- the iron-based carbide contained in the tempered martensite has a plurality of variants, and the iron-based carbide existing in one crystal grain extends in a plurality of directions. Therefore, tempered martensite and lower bainite can be distinguished depending on whether the direction in which the iron-based carbide extends is plural or single.
- the area fraction S ⁇ of retained austenite is expressed by the following equation.
- the area fraction of fresh martensite can be specified by subtracting the area fraction S ⁇ of retained austenite from the area fraction of the remainder in FE-SEM observation.
- Ferrite is a massive crystal grain and does not contain substructure such as lath inside.
- Pearlite is a structure in which ferrite and cementite are alternately layered.
- layered ferrite in pearlite is distinguished from the massive ferrite described above.
- the particle size of carbide means an equivalent circle diameter obtained from the area of the carbide measured on the observation surface of the sample.
- the density and composition of the carbide can be determined, for example, by a transmission electron microscope (TEM) or a three-dimensional atom probe electrolysis ion having an analysis function by energy dispersive X-ray spectroscopy (EDX). It can be measured using a microscope (atom probe field micro ion: AP-FIM).
- the chemical composition of the hot stamping molded body according to the embodiment of the present invention and the steel sheet suitable for the production thereof will be described.
- the hot stamping molded body according to the embodiment of the present invention is manufactured through blanking of a steel plate and at least two quenching of the blanking material. Therefore, the chemical composition of the hot stamped molded product and the steel sheet considers not only the properties of the hot stamped molded product but also these treatments.
- “%”, which is a unit of the content of each element contained in the hot stamped molded body and the steel sheet, means “mass%” unless otherwise specified.
- the hot stamped article according to the present embodiment has C: 0.27% to 0.60%, Mn: 0.50% to 5.00%, Si: 2.00% or less, and P: 0.030% or less.
- C (C: 0.27% to 0.60%) C is inexpensive and greatly contributes to improvement in strength.
- the C content is preferably 0.27% or more, more preferably 0.35% or more, and further preferably 0.40% or more.
- the C content is preferably 0.60% or less.
- Mn 0.50% to 5.00% Mn lowers the Ac3 point and improves the hardenability of the steel sheet. If the Mn content is less than 0.50%, sufficient hardenability may not be obtained. Therefore, the Mn content is preferably 0.50% or more, more preferably 1.00% or more. On the other hand, if the Mn content exceeds 5.00%, the workability of the steel sheet before quenching may deteriorate, and pre-formation before quenching may become difficult. In addition, a band-like structure due to segregation of Mn tends to occur, and the toughness of the steel sheet may deteriorate. Therefore, the Mn content is preferably 5.00% or less.
- Si (Si: 2.00% or less) Si is contained as an impurity in steel, for example.
- the Si content is preferably 2.00% or less, more preferably 1.00% or less. Since Si has the effect
- P 0.030% or less
- P is contained, for example, as an impurity in steel. P deteriorates the workability of the steel sheet or deteriorates the toughness of the hot stamped product. For this reason, the lower the P content, the better. In particular, when the P content exceeds 0.030%, the workability and toughness are significantly reduced. Therefore, the P content is preferably 0.030% or less.
- S is contained as an impurity in steel. S deteriorates the formability of the steel sheet or deteriorates the toughness of the hot stamped product. For this reason, the lower the S content, the better. In particular, when the S content exceeds 0.0100%, the moldability and toughness are significantly reduced. Accordingly, the S content is preferably 0.0100% or less, and more preferably 0.0050% or less.
- sol.Al 0.100% or less
- Al is contained as an impurity in steel. sol. If the Al content exceeds 0.100%, the Ac3 point is excessively high, and the quenching heating may have to be performed above 1200 ° C. Therefore, sol.
- the Al content is preferably 0.100% or less. sol. Since Al has the effect
- N (N: 0.0100% or less) N is contained as an impurity in steel, for example. N deteriorates the formability of the steel sheet. For this reason, the lower the N content, the better. In particular, when the N content exceeds 0.0100%, the moldability is significantly reduced. Therefore, the N content is preferably 0.0100% or less.
- B, Cr, Mo, Ti, Nb, V, Cu, and Ni are optional elements that may be appropriately contained within a predetermined amount in the hot stamped molded body and the steel plate.
- B (B: 0.0000% to 0.0050%) B improves the hardenability of the steel sheet. Therefore, B may be contained. In order to sufficiently obtain this effect, the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the B content is preferably 0.005% or less.
- Cr 0.00% to 0.50%
- Cr improves the hardenability of the steel sheet. Therefore, Cr may be contained.
- the Cr content is preferably 0.18% or more.
- the Cr content is preferably 0.50% or less.
- Mo 0.00% to 0.50%
- Mo improves the hardenability of the steel sheet. Therefore, Mo may be contained. In order to sufficiently obtain this effect, the Mo content is preferably 0.03% or more. On the other hand, if the Mo content exceeds 0.50%, the workability of the steel sheet before quenching may deteriorate, and pre-formation before quenching may become difficult. Therefore, the Mo content is preferably 0.50% or less.
- Ti, Nb, and V are strengthening elements, and contribute to an increase in the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization.
- the Ti content, the Nb content, and the V content are all preferably 0.01% or more.
- the Ti content, the Nb content, or the V content exceeds 0.100%, precipitation of carbonitrides increases and formability may deteriorate. Accordingly, the Ti content, Nb content, and V content are all preferably 0.100% or less.
- both the Cu content and the Ni content are preferably 0.01% or more.
- both Cu content and Ni content are preferably 1.000% or less.
- B 0.0000% to 0.0050%, Cr: 0.00% to 0.50%, Mo: 0.00% to 0.50%, Ti: 0.000% to 0.100%, Nb: 0.000% to 0.100%, V: 0.000% to 0.100%, Cu: 0.000% to 1.000%, or Ni: 0.000% to 1.000%, or Any combination of these is preferable.
- the following elements may be intentionally or unavoidably contained within a predetermined amount in the hot stamping body and the steel plate. That is, O: 0.001% to 0.02%, W: 0.001% to 0.1%, Ta: 0.001% to 0.1%, Sn: 0.001% to 0.05%, Sb: 0.001% to 0.05%, As: 0.001% to 0.05%, Mg: 0.0001% to 0.05%, Ca: 0.001% to 0.05%, Y: 0.001% to 0.05%, Zr: 0.001% to 0.05%, La 0.001% to 0.05%, or Ce: 0.001% to 0.05%, or any of these A combination may be established.
- a tensile strength of 1900 MPa or more can be obtained, and even when a low stress fracture occurs, the stress at which the fracture occurs can be 1800 MPa or more.
- the vehicle body can be reduced in weight while obtaining excellent collision safety.
- the collision safety is a neck characteristic of the plate thickness and the collision safety.
- the plate thickness can be reduced to 1/5 by increasing the tensile strength by five times. This reduction in plate thickness has a great effect on reducing the weight of the automobile and improving the fuel consumption.
- a method for manufacturing a hot stamping molded body according to an embodiment of the present invention will be described.
- a blank material is formed from a steel plate having the above-described chemical composition, the blank material is subjected to at least twice quenching, and one or both of the two times quenching is performed.
- the blank is formed with
- the first quenching is mainly performed in order to make the average particle size of the carbide in the hot stamping molded product 0.5 ⁇ m or less.
- the ratio of bainite, fresh martensite and tempered martensite which are likely to contain fine carbides is high, and the ratio of ferrite and pearlite which is likely to contain coarse carbides is low.
- the total area fraction of bainite, fresh martensite and tempered martensite is preferably 80% or more. Bainite, fresh martensite and tempered martensite are also called low-temperature transformation structures, and steel structures containing 80% or more of these are very fine.
- the number density of carbides in the steel sheet after the first heat treatment is preferably 0.50 pieces / ⁇ m 2 or more. This is because the carbide that becomes the nucleation site of the reverse transformation to ⁇ is finely dispersed during the heating of the second heat treatment, and the old ⁇ particle size after the second heat treatment (the old ⁇ particle size in the hot stamped product) This is to make the thickness of 20 ⁇ m or less easier.
- carbonized_material in a hot stamping body it is preferable that the average particle diameter of the carbide
- a blank is formed by blanking a steel plate by shear cutting or punching.
- the Vickers hardness of the steel plate used in the present embodiment is, for example, 500 Hv or less, preferably 450 Hv or less. If the Vickers hardness is 500 Hv or less, blanking can be easily performed. Moreover, according to this embodiment, even if the Vickers hardness of a steel plate is 500 Hv or less, sufficient strength, for example, tensile strength of 1900 MPa or more can be obtained.
- first quenching First heat treatment
- the blank is heated to a first temperature of (Ac3 point ⁇ 50) ° C. to 1200 ° C. at an average heating rate of 2 ° C./second or more, and the blank is heated from the first temperature to 250 ° C. or less. Cool to the second temperature.
- the first temperature is (Ac3 point ⁇ 50 ° C.), preferably 900 ° C. or higher, more preferably 1000 ° C. or higher.
- the first temperature is 1200 ° C. or lower.
- the average heating rate up to the first temperature is 2 ° C./second or more, preferably 5 ° C./second or more, more preferably 10 ° C./second or more, and further preferably 100 ° C./second or more. is there.
- the heating method is not particularly limited, and examples thereof include atmospheric heating, electric heating, and infrared heating.
- the holding time is preferably 1 second or longer, more preferably 100 seconds or longer.
- the holding time is preferably 600 seconds or less.
- the second temperature which is the cooling stop temperature
- the second temperature exceeds 250 ° C.
- ferrite and pearlite that easily contain coarse carbides are easily generated, and a low-temperature transformation structure that easily contains fine carbides is hardly generated. Therefore, the second temperature is 250 ° C. or lower.
- the average cooling rate is preferably 10 ° C./second or more. This is to avoid ferrite transformation and pearlite transformation.
- air cooling accompanying the transportation of the blank material may be performed.
- the cooling method is not particularly limited, and examples thereof include gas cooling and water cooling.
- gas cooling or water cooling it is preferable to give tension to the blank material so that the blank material is not deformed by thermal stress.
- the blank material may be pressed with a flat mold and the first heat treatment may be finished in a flat plate state, or the mold having the shape of a hot stamping body may be used during the first heat treatment.
- a blank material may be pressed. You may process into the shape of a hot stamping molded object, dividing into two steps, the 1st heat processing and the 2nd heat processing.
- the Ac3 point (° C.) can be calculated by the following formula.
- [X] indicates the content (% by mass) of the element X.
- Ac3 point 910 ⁇ 203 ⁇ [C] ⁇ 30 [Mn] ⁇ 11 [Cr] +44.7 [Si] +400 [Al] +700 [P] -15.2 [Ni] -20 [Cu] +400 [Ti] +104 [V] +31.5 [Mo]
- the blank material is heated from the second temperature to a third temperature of (Ac3 point ⁇ 50) ° C. to 1200 ° C. at an average heating rate of 2 ° C./second or more. Cool from temperature to a fourth temperature of 250 ° C. or lower.
- the third temperature is (Ac3 point ⁇ 50 ° C.) or higher, preferably (Ac3 point ⁇ 20 ° C.) or higher, and more preferably Ac3 point or higher.
- the third temperature is 1200 ° C. or lower, preferably 1000 ° C. or lower, more preferably 900 ° C. or lower, and further preferably 850 ° C. or lower.
- the average heating rate up to the third temperature is 2 ° C./second or more, preferably 5 ° C./second or more, more preferably 10 ° C./second or more, and further preferably 100 ° C./second or more. is there.
- the heating method is not particularly limited, and examples thereof include atmospheric heating, electric heating, and infrared heating. If the shape of the blank material after the first heat treatment is flat, electric heating is most preferable among the above three types. This is because electric heating can achieve the highest rate of temperature increase. In the case where molding is performed during the first heat treatment, infrared heating is most preferable among the above three types. This is because it is difficult to heat the formed blank material evenly by electric heating, and infrared heating can achieve a higher temperature increase rate than atmospheric heating.
- the holding time is preferably 0.1 seconds or longer.
- the holding time is preferably 300 seconds or shorter, more preferably 30 seconds or shorter.
- the fourth temperature which is the cooling stop temperature
- the fourth temperature exceeds 250 ° C., quenching is insufficient, and the hot stamped molded article has insufficient martensite. Therefore, the fourth temperature is 250 ° C. or lower, preferably Ms point (° C.) ⁇ 50 ° C. or lower.
- the average cooling rate is preferably 20 ° C./second or more.
- the average cooling rate in the temperature range from 700 ° C. to Ms point ⁇ 50 ° C. is less than 20 ° C./second, ferrite transformation, pearlite transformation or bainite transformation occurs, and the total area fraction of fresh martensite and tempered martensite May be less than 80%. Therefore, the average cooling rate in the temperature range from 700 ° C. to Ms point ⁇ 50 ° C. is preferably 20 ° C./second or more.
- the Ms point (° C.) can be calculated by the following formula.
- [X] indicates the content (% by mass) of the element X.
- Ms point 539-423 [C] -30.4 [Mn] -17.7 [Ni] -12. 1 [Cr]-7.5 [Mo]
- the upper limit of the cooling rate from the third temperature to the fourth temperature is not particularly limited, but even if a special apparatus for cooling is used, the cooling rate is usually 2000 ° C./second or less industrially.
- the cooling rate is generally 1000 ° C./second or less for simple water cooling and 500 ° C./second or less for simple mold cooling.
- the upper limit of the cooling rate in the cooling from the first temperature to the second temperature is the same.
- the cooling of the blank material from the third temperature to the fourth temperature is performed in the mold.
- the blank material may be cooled by heat removal from the mold, or the blank material may be cooled by spraying water on the blank material in the mold.
- the hot stamping molded body according to the embodiment of the present invention can be manufactured.
- the hot stamping molded body may be subjected to heating at a temperature of 50 ° C. to 650 ° C. within 6 hours.
- the heating temperature is 50 ° C. to 400 ° C.
- fine carbides are precipitated in the martensite during the heating, and the hydrogen embrittlement characteristics are improved.
- the heating temperature is 400 to 650 ° C., alloy carbides and / or intermetallic compounds are precipitated during the heating, and the strength increases due to particle dispersion strengthening.
- the time from the end of the first quenching to the start of the second quenching is not particularly limited, but depending on the composition of the blank material, fine carbides in the blank material grow by holding at room temperature for a long time.
- the average particle size of the carbide after quenching may increase. For this reason, the time is preferably within one month, more preferably within one week, and even more preferably within one day.
- the first quenching or the second quenching or both may be repeated twice or more.
- the old ⁇ grain size of the hot stamped molded product tends to decrease.
- the old ⁇ particle size is preferably 15 ⁇ m or less, and more preferably 10 ⁇ m or less.
- an older ⁇ particle size of 15 ⁇ m or less or 10 ⁇ m or less is easily obtained.
- a hot-rolled steel plate suitable for manufacturing a hot stamped molded body As a steel plate suitable for manufacturing a hot stamped body, a hot-rolled steel plate that has not been annealed, a hot-rolled steel plate that has been annealed to a hot-rolled steel plate, a hot-rolled steel plate, or a hot-rolled annealed steel plate has been subjected to cold rolling. Either a cold-rolled steel sheet as cold-rolled or a cold-rolled annealed steel sheet obtained by annealing a cold-rolled steel sheet may be used.
- a steel having the above chemical composition is melted by a conventional method and continuously cast to obtain a slab.
- Steel may be cast to obtain a steel ingot, and the steel ingot may be rolled into pieces to obtain a steel piece. From the viewpoint of productivity, continuous casting is preferable.
- the casting speed of continuous casting is preferably less than 2.0 m / min in order to effectively suppress Mn center segregation and V-shaped segregation. Moreover, in order to keep the cleanness of the surface of a slab favorable and to ensure productivity, the casting speed is preferably set to 1.2 m / min or more.
- the slab heating temperature is set to 1100 ° C. or higher and the finishing temperature is set to 850 ° C. or higher for solutionization of inclusions.
- the coiling temperature is preferably 500 ° C. or higher from the viewpoint of workability, and 650 ° C. or lower from the viewpoint of suppressing a decrease in yield due to scale generation.
- the hot-rolled steel sheet obtained by hot rolling is descaled by pickling or the like.
- the hot-rolled steel sheet after descaling can be used for the production of a hot stamping body.
- the hot-rolled steel sheet may be subjected to hot-rolled sheet annealing after descaling.
- a hot-rolled annealed steel sheet obtained by hot-rolled sheet annealing can also be used for manufacturing a hot stamped molded body.
- the hot-rolled annealed steel sheet may be cold-rolled after the hot-rolled sheet annealing.
- a cold-rolled steel sheet obtained by cold rolling can be used for producing a hot stamping body.
- Cold rolling may be performed by a conventional method.
- the rolling reduction in the cold rolling is preferably 30% or more from the viewpoint of securing a good flatness, and preferably 80% or less in order to avoid an excessive load.
- Cold-rolled steel sheet may be subjected to cold-rolled sheet annealing.
- a cold-rolled annealed steel sheet obtained by cold-rolled sheet annealing can be used for manufacturing a hot stamped molded body.
- annealing may be performed after performing a treatment such as degreasing according to a conventional method as necessary.
- the annealing is preferably performed in a continuous annealing line.
- it is preferable to soak in a temperature range from Ac3 point to (Ac3 point + 100 ° C) and below for 1 second to 1000 seconds, and then to a temperature range from 250 ° C to 550 ° C. It is preferable to hold for 1 to 30 minutes.
- hot-dip zinc-based plating is preferably performed in a continuous hot-dip galvanizing line.
- annealing may be performed prior to hot dip galvanizing, or the soaking temperature may be lowered and galvanizing may be performed without annealing.
- An alloying treatment may be performed after hot dip galvanizing to form an alloyed hot dip galvanized steel sheet.
- Zinc-based plating may be performed by electroplating.
- Examples of the zinc-based plating include hot dip galvanizing, alloying hot dip galvanizing, electrogalvanizing, hot dip zinc-aluminum alloy plating, electric nickel-zinc alloy plating, and electric iron-zinc alloy plating.
- the adhesion amount of plating is not particularly limited, and may be approximately the same as the adhesion amount of a conventional plated steel sheet.
- Zinc-based plating can be applied to at least a part of the surface of the steel material, but in general, zinc-based plating of a steel sheet is applied to one or both surfaces of the steel sheet.
- a cold-rolled steel plate, an aluminum-plated steel plate, a hot-dip galvanized steel plate and an alloyed hot-dip galvanized steel plate were produced from a hot-rolled steel plate having a thickness of 3.2 mm as follows. First, a hot-rolled steel sheet having a thickness of 3.2 mm is subjected to hot-rolling sheet annealing at 600 ° C. for 2 hours and cold-rolling with a reduction rate of 50% to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. It was. Next, some of the cold-rolled steel sheets were annealed in a continuous melt annealing facility or a continuous melt plating facility.
- the cold-rolled steel sheet was held at 800 ° C. for 120 seconds and then held at 400 ° C. for 200 seconds.
- hot-dip aluminum plating, hot-dip galvanizing, or alloying hot-dip galvanizing was performed on the cold-rolled steel sheet at a temperature of 500 ° C. or lower. In this way, hot-rolled steel sheets, cold-rolled steel sheets, aluminum-plated steel sheets, hot-dip galvanized steel sheets, and galvannealed steel sheets were prepared as hot stamping steel sheets.
- first heat treatment first quenching
- second quenching second heat treatment
- Tables 2 and 3 show conditions for the first heat treatment and conditions for the second heat treatment.
- first heat treatment atmosphere heating was performed, air cooling was performed from the holding temperature to 700 ° C., and cooling was performed at an average cooling rate of 50 ° C./second in a flat plate mold from 700 ° C. to the cooling stop temperature.
- second heat treatment atmosphere heating was performed when the heating rate was 50 ° C./second or less, and electric heating was performed when the heating rate was higher than 50 ° C./second.
- the steel structure after the first heat treatment and before the second heat treatment and the steel structure after the second heat treatment were observed.
- the steel structure observation method is as described above.
- the tensile test piece based on JISZ2201 was extract
- Test No. 1 since the holding temperature of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, the average particle size of the carbide was excessive, and sufficient tensile strength could not be obtained.
- Test No. 6 since the first quenching was not performed, the old ⁇ particle size of the hot stamped molded product was insufficient, the average particle size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was not obtained. .
- Test No. In No. 7 since the cooling stop temperature of the first quenching was too high, the old ⁇ particle size of the hot stamped product was insufficient, the average particle size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was achieved. It was not obtained.
- Test No. 17 since the average heating rate of the first quenching was too low, the old ⁇ grain size of the hot stamped molded product was insufficient, low stress fracture occurred, and sufficient tensile strength could not be obtained.
- Test No. 18 since the holding temperature of the first quenching was too low, the old ⁇ grain size of the hot stamped product was insufficient, the average grain size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was obtained. I could't. Test No. In No. 19, since the average heating rate of the second quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, low stress fracture occurred, and sufficient tensile strength could not be obtained. Test No. In No. 20, since the cooling stop temperature of the second quenching was too high, the total area fraction of fresh martensite and tempered martensite was insufficient, and sufficient tensile strength could not be obtained.
- Test No. 23 since the holding temperature of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, the average particle size of the carbide was excessive, and sufficient tensile strength could not be obtained.
- Test No. 28 since the holding temperature of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, the average particle size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was obtained. I could't. Test No. In No. 29, since the first quenching was not performed, the old ⁇ particle size of the hot stamped molded product was insufficient, the average particle size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was not obtained. . Test No.
- Test No. 41 since the average heating rate of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, resulting in low stress fracture, and sufficient tensile strength could not be obtained.
- Test No. 42 since the holding temperature of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, the average particle size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was obtained. I could't.
- Test No. In No. 43 since the cooling stop temperature of the first quenching was too high, the average particle size of the carbide of the hot stamped article was excessive, resulting in low stress fracture, and sufficient tensile strength could not be obtained. Test No. In No.
- Test No. 51 since the average heating rate of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, resulting in low stress fracture, and sufficient tensile strength could not be obtained.
- Test No. 52 since the holding temperature of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, the average particle size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was obtained. I could't. Test No. In No. 53, since the cooling stop temperature of the first quenching was too high, the average particle size of the carbide in the hot stamped article was excessive, resulting in low stress fracture, and sufficient tensile strength could not be obtained. Test No. In No.
- Test No. 64 since the average heating rate of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, resulting in low stress fracture, and sufficient tensile strength could not be obtained.
- Test No. 65 since the holding temperature of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, the average particle size of the carbide was excessive, low stress fracture occurred, and sufficient tensile strength was obtained. I could't.
- Test No. In No. 66 since the cooling stop temperature of the first quenching was too high, the average particle size of the carbide of the hot stamped article was excessive, resulting in low stress fracture, and sufficient tensile strength could not be obtained. Test No. In No. 64, since the average heating rate of the first quenching was too low, the old ⁇ particle size of the hot stamped product was insufficient, resulting in low stress fracture, and sufficient tensile strength could not be obtained.
- Test No. In No. 65 since the holding temperature of the first quen
- test No. 1 in the first experiment was performed. 10, no. 31, no. 37, no. 47 and no.
- a blank was formed in the same manner as in No. 58, and the blank was subjected to first quenching (first heat treatment), second quenching (second heat treatment), and third quenching (third heat treatment).
- Table 6 shows conditions for the first heat treatment, conditions for the second heat treatment, and conditions for the third heat treatment.
- atmosphere heating was performed when the heating rate was 50 ° C./second or less, and electric heating was performed when the heating rate exceeded 50 ° C./second. From the holding temperature to 700 ° C., air cooling was performed, and from 700 ° C. to the cooling stop temperature, cooling was performed at an average cooling rate of 100 ° C./sec. In this way, various hot stamping molded articles were produced.
- the present invention can be used, for example, in industries related to hot stamped molded articles suitable for automobile parts.
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Abstract
Description
dσ/dε=σ ・・・(式1)
鋼板からブランク材を形成する工程と、
前記ブランク材の第1の焼入れを行う工程と、
前記第1の焼入れの後に、前記ブランク材の第2の焼入れを行う工程と、
を有し、
前記第1の焼入れを行う工程は、
前記ブランク材を(Ac3点-50)℃以上1200℃以下の第1の温度まで2℃/秒以上の平均加熱速度で加熱する工程と、
前記ブランク材を前記第1の温度から250℃以下の第2の温度まで冷却する工程と、
を有し、
前記第2の焼入れを行う工程は、
前記ブランク材を前記第2の温度から(Ac3点-50)℃以上1200℃以下の第3の温度まで2℃/秒以上の平均加熱速度で加熱する工程と、
前記ブランク材を前記第3の温度から250℃以下の第4の温度まで冷却する工程と、
を有し、
前記第1の焼入れ若しくは前記第2の焼入れ又はこれらの両方において前記ブランク材の成形を行うことを特徴とするホットスタンプ成形体の製造方法。
前記第1の温度まで加熱する工程と前記第2の温度まで冷却する工程との間に、前記第1の温度に1秒間以上保持する工程を有することを特徴とする(1)に記載のホットスタンプ成形体の製造方法。
前記第3の温度は、(Ac3点-50)℃以上1000℃以下であることを特徴とする(1)又は(2)に記載のホットスタンプ成形体の製造方法。
前記第2の温度から前記第3の温度までの加熱を5℃/秒以上の平均加熱速度で行うことを特徴とする(1)~(3)のいずれかに記載のホットスタンプ成形体の製造方法。
前記第3の温度まで加熱する工程と前記第4の温度まで冷却する工程との間に、前記第3の温度に0.1秒以上300秒以下保持する工程を有することを特徴とする(1)~(4)のいずれかに記載のホットスタンプ成形体の製造方法。
前記第2の焼入れを行う工程は、700℃からMs点-50℃の第5の温度まで20℃/秒の平均冷却速度で前記ブランク材を冷却する工程を有することを特徴とする(1)~(5)のいずれかに記載のホットスタンプ成形体の製造方法。
フレッシュマルテンサイト及び焼戻しマルテンサイトの面積分率:合計で80%以上、
旧オーステナイト粒径:20μm以下、かつ
炭化物の平均粒径:0.5μm以下、
で表される鋼組織を有することを特徴とするホットスタンプ成形体。
C含有量が0.27質量%以上0.60質量%以下であることを特徴とする(7)に記載のホットスタンプ成形体。
ビッカース硬さが550Hv以上であることを特徴とする(7)又は(8)に記載のホットスタンプ成形体。
フレッシュマルテンサイト及び焼戻しマルテンサイトは強度の向上に寄与する。フレッシュマルテンサイト及び焼戻しマルテンサイトの面積分率が合計で80%未満では十分な強度、例えば1900MPa以上の引張強度が得られない。従って、フレッシュマルテンサイト及び焼戻しマルテンサイトの面積分率は合計で80%以上である。材料の機械的特性は組織又は相の体積分率に依存するが、鋼組織が等方的であれば、体積分率は面積分率と等価である。そして、面積分率は体積分率よりも簡易に測定することができる。そこで、本願では、面積分率を用いる。
旧γ粒径は、旧γ粒の平均粒径である。旧γ粒径が20μm超では、十分な破壊靭性が得られず、低応力破壊が生じやすい。従って、旧γ粒径は20μm以下である。破壊靱性の向上及び低応力破壊の抑制の観点から、旧γ粒径は、好ましくは15μm以下であり、より好ましくは10μm以下である。
炭化物の平均粒径が0.5μm超では、粗大な炭化物を起点とした低応力破壊が生じやすい。従って、炭化物の平均粒径は0.5μm以下である。低応力破壊の抑制の観点から、炭化物の平均粒径は、好ましくは0.3μm以下である。炭化物には、セメンタイト及びε炭化物等の鉄系炭化物並びに炭窒化物が含まれる。
Sγ=(I200f+I220f+I311f)/(I200b+I211b)×100
(I200f、I220f、I311fは、それぞれ面心立方格子(fcc)相の(200)、(220)、(311)の回折ピークの強度、I200b、I211bは、それぞれ体心立方格子(bcc)相の(200)、(211)の回折ピークの強度を示す。)
Cは、安価で強度の向上に大きく寄与する。C含有量が0.27%未満では、高価な元素が含有されていなければ、十分な強度、例えば1900MPa以上の強度を得にくい。従って、C含有量は、好ましくは0.27%以上であり、より好ましくは0.35%以上であり、更に好ましくは0.40%以上である。一方、C含有量が0.60%超では、水素脆化特性が大きく劣化することがある。従って、C含有量は、好ましくは0.60%以下である。
Mnは、Ac3点を低下させて鋼板の焼入れ性を向上する。Mn含有量が0.50%未満では、十分な焼入れ性が得られないことがある。従って、Mn含有量は、好ましくは0.50%以上であり、より好ましくは1.00%以上である。一方、Mn含有量が5.00%超では、焼入れ前の鋼板の加工性が劣化することがあり、焼入れ前の予成形が困難になることがある。また、Mnの偏析に起因したバンド状組織が生じやすくなり、鋼板の靭性が劣化することがある。従って、Mn含有量は、好ましくは5.00%以下である。
Siは、例えば鋼中に不純物として含有される。Si含有量が2.00%超では、Ac3点が過度に高く、焼入れの加熱を1200℃超で行わなければならなかったり、鋼板の化成処理性及び亜鉛めっきのめっき性が低下したりすることがある。従って、Si含有量は、好ましくは2.00%以下であり、より好ましくは1.00%以下である。Siは鋼板の焼入れ性を高める作用を有するため、Siが含有されていてもよい。
Pは、例えば鋼中に不純物として含有される。Pは、鋼板の加工性を劣化させたり、ホットスタンプ成形体の靱性を劣化させたりする。このため、P含有量は低ければ低いほどよい。特に、P含有量が0.030%超で、加工性及び靱性の低下が著しい。従って、P含有量は、好ましくは0.030%以下である。
Sは、例えば鋼中に不純物として含有される。Sは、鋼板の成形性を劣化させたり、ホットスタンプ成形体の靱性を劣化させたりする。このため、S含有量は低ければ低いほどよい。特に、S含有量が0.0100%超で、成形性及び靱性の低下が著しい。従って、S含有量は、好ましくは0.0100%以下であり、より好ましくは0.0050%以下である。
sol.Alは、例えば鋼中に不純物として含有される。sol.Al含有量が0.100%超では、Ac3点が過度に高く、焼入れの加熱を1200℃超で行わなければならないことがある。従って、sol.Al含有量は、好ましくは0.100%以下である。sol.Alは、脱酸により鋼を健全化する作用を有するため、sol.Alが含まれていてもよい。
Nは、例えば鋼中に不純物として含有される。Nは、鋼板の成形性を劣化させる。このため、N含有量は低ければ低いほどよい。特に、N含有量が0.0100%超で、成形性の低下が著しい。従って、N含有量は、好ましくは0.0100%以下である。
Bは、鋼板の焼入れ性を向上する。従って、Bが含有されていてもよい。この効果を十分に得るために、B含有量は、好ましくは0.0001%以上である。その一方で、B含有量が0.0050%超では、上記の作用による効果は飽和して、コスト的に不利となる。従って、B含有量は、好ましくは0.005%以下である。
Crは、鋼板の焼入れ性を向上する。従って、Crが含有されていてもよい。この効果を十分に得るために、Cr含有量は、好ましくは0.18%以上である。その一方で、Cr含有量が0.50%超では、焼入れ前の鋼板の加工性が劣化することがあり、焼入れ前の予成形が困難になることがある。従って、Cr含有量は、好ましくは0.50%以下である。
Moは、鋼板の焼入れ性を向上する。従って、Moが含有されていてもよい。この効果を十分に得るために、Mo含有量は、好ましくは0.03%以上である。その一方で、Mo含有量が0.50%超では、焼入れ前の鋼板の加工性が劣化することがあり、焼入れ前の予成形が困難になることがある。従って、Mo含有量は、好ましくは0.50%以下である。
Ti、Nb及びVは、強化元素であり、析出物強化、フェライト結晶粒の成長抑制による細粒強化及び再結晶の抑制を通じた転位強化にて、鋼板の強度の上昇に寄与する。この効果を十分に得るために、Ti含有量、Nb含有量及びV含有量は、いずれも好ましくは0.01%以上である。その一方で、Ti含有量、Nb含有量又はV含有量が0.100%超では、炭窒化物の析出が多くなり成形性が劣化することがある。従って、Ti含有量、Nb含有量及びV含有量は、いずれも好ましくは0.100%以下である。
Cu及びNiは、強度の向上に寄与する。この効果を十分に得るために、Cu含有量及びNi含有量は、いずれも好ましくは0.01%以上である。その一方で、Cu含有量又はNi含有量が1.000%超では、酸洗性、溶接性及び熱間加工性等が劣化することがある。従って、Cu含有量及びNi含有量は、いずれも好ましくは1.000%以下である。
鋼板をシャー切断又は打ち抜き加工等によりブランキングしてブランク材を形成する。本実施形態で用いる鋼板のビッカース硬さは、例えば500Hv以下であり、好ましくは450Hv以下である。ビッカース硬さが500Hv以下であれば、ブランキングを容易に行うことができる。また、本実施形態によれば、鋼板のビッカース硬さが500Hv以下であっても、十分な強度、例えば1900MPa以上の引張強度を得ることができる。
1回目の熱処理では、ブランク材を(Ac3点-50)℃以上1200℃以下の第1の温度まで2℃/秒以上の平均加熱速度で加熱し、ブランク材を第1の温度から250℃以下の第2の温度まで冷却する。
Ac3点=910-203√[C]-30[Mn]-11[Cr]+44.7[Si]
+400[Al]+700[P]-15.2[Ni]-20[Cu]
+400[Ti]+104[V]+31.5[Mo]
2回目の熱処理では、ブランク材を第2の温度から(Ac3点-50)℃以上1200℃以下の第3の温度まで2℃/秒以上の平均加熱速度で加熱し、ブランク材を第3の温度から250℃以下の第4の温度まで冷却する。
Ms点=539-423[C]-30.4[Mn]-17.7[Ni]
-12.1[Cr]-7.5[Mo]
表1に示す化学組成を有するスラブに熱間圧延を施した。熱間圧延では、スラブ加熱温度を1250℃とし、仕上げ温度を930℃とし、巻き取り温度を650℃とした。仕上げ温度(930℃)から巻き取り温度(650℃)までの冷却では、平均冷却速度を20℃/秒とした。このようにして、厚さが1.6mm又は3.2mmの熱延鋼板を得た。次いで、熱延鋼板の脱スケール処理を行った。表1に示す化学組成の残部はFe及び不純物である。表1中の下線は、その数値が本発明の範囲から外れていることを示す。
Δσa/Δεa=σa ・・・(式2)
第2の実験では、第1の実験における試験No.10、No.31、No.37、No.47及びNo.58と同様にしてブランク材を形成し、ブランク材の第1の焼入れ(1回目の熱処理)、第2の焼入れ(2回目の熱処理)及び第3の焼入れ(3回目の熱処理)を行った。1回目の熱処理の条件、2回目の熱処理の条件及び3回目の熱処理の条件を表6に示す。表6に示すように、3回目の熱処理では、加熱速度が50℃/秒以下の場合は雰囲気加熱を行い、50℃/秒超の場合は電気加熱を行った。保持温度から700℃までは空冷し、700℃から冷却停止温度までは金型内でプレス成形しながら100℃/秒の平均冷却速度で冷却した。このようにして、種々のホットスタンプ成形体を製造した。
Claims (9)
- 鋼板からブランク材を形成する工程と、
前記ブランク材の第1の焼入れを行う工程と、
前記第1の焼入れの後に、前記ブランク材の第2の焼入れを行う工程と、
を有し、
前記第1の焼入れを行う工程は、
前記ブランク材を(Ac3点-50)℃以上1200℃以下の第1の温度まで2℃/秒以上の平均加熱速度で加熱する工程と、
前記ブランク材を前記第1の温度から250℃以下の第2の温度まで冷却する工程と、
を有し、
前記第2の焼入れを行う工程は、
前記ブランク材を前記第2の温度から(Ac3点-50)℃以上1200℃以下の第3の温度まで2℃/秒以上の平均加熱速度で加熱する工程と、
前記ブランク材を前記第3の温度から250℃以下の第4の温度まで冷却する工程と、
を有し、
前記第1の焼入れ若しくは前記第2の焼入れ又はこれらの両方において前記ブランク材の成形を行うことを特徴とするホットスタンプ成形体の製造方法。 - 前記第1の温度まで加熱する工程と前記第2の温度まで冷却する工程との間に、前記第1の温度に1秒間以上保持する工程を有することを特徴とする請求項1に記載のホットスタンプ成形体の製造方法。
- 前記第3の温度は、(Ac3点-50)℃以上1000℃以下であることを特徴とする請求項1又は2に記載のホットスタンプ成形体の製造方法。
- 前記第2の温度から前記第3の温度までの加熱を5℃/秒以上の平均加熱速度で行うことを特徴とする請求項1乃至3のいずれか1項に記載のホットスタンプ成形体の製造方法。
- 前記第3の温度まで加熱する工程と前記第4の温度まで冷却する工程との間に、前記第3の温度に0.1秒以上300秒以下保持する工程を有することを特徴とする請求項1乃至4のいずれか1項に記載のホットスタンプ成形体の製造方法。
- 前記第2の焼入れを行う工程は、700℃からMs点-50℃の第5の温度まで20℃/秒の平均冷却速度で前記ブランク材を冷却する工程を有することを特徴とする請求項1乃至5のいずれか1項に記載のホットスタンプ成形体の製造方法。
- フレッシュマルテンサイト及び焼戻しマルテンサイトの面積分率:合計で80%以上、
旧オーステナイト粒径:20μm以下、かつ
炭化物の平均粒径:0.5μm以下、
で表される鋼組織を有することを特徴とするホットスタンプ成形体。 - C含有量が0.27質量%以上0.60質量%以下であることを特徴とする請求項7に記載のホットスタンプ成形体。
- ビッカース硬さが550Hv以上であることを特徴とする請求項7又は8に記載のホットスタンプ成形体。
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Also Published As
Publication number | Publication date |
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US11505846B2 (en) | 2022-11-22 |
JPWO2018134874A1 (ja) | 2019-11-07 |
EP3572536B1 (en) | 2022-11-30 |
MX2019007946A (es) | 2019-08-29 |
EP3572536A4 (en) | 2020-08-05 |
JP6795042B2 (ja) | 2020-12-02 |
KR20190093613A (ko) | 2019-08-09 |
CA3050217A1 (en) | 2018-07-26 |
KR102262353B1 (ko) | 2021-06-08 |
CN110168116B (zh) | 2022-02-18 |
CN110168116A (zh) | 2019-08-23 |
ES2935623T3 (es) | 2023-03-08 |
BR112019013393A2 (pt) | 2020-03-03 |
EP3572536A1 (en) | 2019-11-27 |
US20190330711A1 (en) | 2019-10-31 |
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